1. Field of the Invention
This invention relates to DC-to-DC converters, DC-to-AC inverters and AC-to-DC converters. The major characteristic of this power conversion technique is that primary switching elements switches at zero voltage and the secondary rectifiers means have negligible reverse recovery losses.
2. Description of the Prior Art
There is a continuing industry demand for increasing power density, which means more power transferred in a given volume. A method for increasing the power transfer through the converter is to increase the switching frequency in order to minimize the size of magnetic and the capacitors. Using prior art topologies such as forward or flyback, which employ “hard” switching techniques makes high frequency operation less efficient. The switching losses associated with switching elements, which turn on when there is a voltage across them, are proportional with the switching frequency. An increase in switching frequency leads to an increase in switching losses and an increase in the level of electromagnetic interference (EMI).
In order to overcome limitations in switching speeds, the prior art has devised a new family of soft transition. The U.S. Pat. Nos. 5,132,889, 5,126,931, 5,231,563, 5,434,768 present several methods of accomplishing zero voltage switching across the primary switches.
Another power loss mechanism is due to the reverse recovery in the output rectifiers. During switching when a negative polarity voltage is applied to a rectifier in conduction the current through the rectifier will continue to conduct until all the carriers in the rectifier's junctions are depleted. During this period of time the current polarity will reverse, the current flowing from the cathode to the anode, while the voltage across the diode is still positive from the anode to the cathode. The current flowing in reverse through the diode will reach a peak value referred in literature as Irrm. Further on, while the rectifiers' junction is depleting the carriers, the rectifier becomes a high impedance device. The current through the rectifier will decrease rapidly from Irrm level to zero. During the same time the negative voltage across the rectifier will build up to high levels.
During the period of time when there is a negative voltage across the diode and negative current is flowing through it, there will be power dissipation in the device. This kind of loss is referred in the literature as reverse recovery losses. The reverse recovery loss is proportional with the reverse recovery current Irrm, the negative voltage across the rectifier and the frequency.
The reverse recovery current Irrm, which is a key component in reverse recovery loss, is a function of this type of device, the temperature and the current slope at turn off. The reverse recovery characteristics are getting worse for higher voltage rectifiers. As a result the reverse recovery loss becomes a significant loss mechanism for higher output voltage applications. The reverse recovery current Irrm is direct dependent of the current slope at turn off. A soft slope reduces the reverse recovery current and as a consequence reduces the reverse recovery loss. To accomplish a very soft slope current at turn off an inductive element has to be in series with the rectifier. The inductor element will prevent a fast current variation dI/dt. The presence of an inductive element in series with the rectifier will increase the negative voltage across the rectifier at turn off. The reverse voltage across the rectifier can reach very high levels and can exceed the voltage break down of the device, leading to failure.
RC snubbers or complicated lossless snubers can be added across the rectifier to reduce the reverse recovery loss and the voltage stress on the devices. This leads to complex circuits and which negatively affects the efficiency and the reliability. As a result of these limitations the high voltage converters have to operate at lower frequency in order to reduce the power dissipation associated with reverse recovery.
What is needed is a converter topology which can operate at constant frequency with zero voltage switching on the primary switches and soft commutation on the output rectifiers, wherein low current slope through the rectifiers at turn off is associated with low negative voltage across the rectifiers. The lowest voltage across the output rectifiers in a DC-DC converter is the output voltage. As a result our goal is to reduce the negative voltage across the output rectifier to the level of the output voltage.